An image projection apparatus introduces light from a light source to a spatial light modulating element such as a liquid crystal panel or a micro-mirror array device through an illumination optical system, and then projects light image-modulated by the spatial light modulating element onto a projection surface such as a screen to display an image.
Japanese Patent Laid-Open No. 3-111806 discloses an illumination optical system for such an image projection apparatus. The illumination optical system includes an integration optical system which includes two lens arrays (first and second lens arrays). The integration optical system divides light from a light source into plural light fluxes by plural collective lenses constituting the first lens array, each collective lens having a rectangular aperture. The integration optical system overlaps the plural light fluxes with each other on an illumination surface (spatial light modulating element) by the second lens array formed of plural collective lenses corresponding to the plural rectangular apertures of the first lens array and a collective lens (condenser lens) disposed closer to the illumination surface than the second lens array. This integration optical system provides a homogeneous intensity distribution of the light illuminating the illumination surface.
Japanese Patent Laid-Open No. 2001-337393 discloses an illumination optical system suppressing horizontal color unevenness in a projected image, the color unevenness being generated due to a wavelength selective dichroic mirror. In this illumination optical system, a principal point of a collection optical system (plural collective lenses) which collects light from an integration optical system constituted by first and second lens arrays onto a spatial light modulating element is located at an intermediate position between the second lens array and the spatial light modulating element. This illumination optical system can improve a degree of parallelization of light entering the dichroic mirror as compared with conventional illumination optical systems, resulting in reduction in effect of an angular characteristic of the dichroic mirror to suppress the horizontal color unevenness in the projected image.
Japanese Patent Laid-Open No. 2004-45907 discloses an illumination optical system including an aperture stop which limits a spread angle of light entering a polarization beam splitter (hereinafter referred to as a “PBS”). The aperture stop increases an F-number of the illumination optical system, achieving a high contrast.
It is important for image projection apparatuses to have capability of increasing brightness and contrast of a projected image. However, it is difficult in reality to increase both the brightness and the contrast. The increase of the brightness needs increase of light utilization efficiency. The increase of the light utilization efficiency is realized by reducing the F-number of the illumination optical system. However, the reduction of the F-number reduces the contrast.
The contrast is reduced due to deterioration of reflection performance of the PBS which is used as a color separation/combination element, the deterioration being caused by increase of an incident angle of light entering the PBS. Reducing the F-number to increase the brightness increases the incident angle of the light entering the PBS, which deteriorates the reflection performance of the PBS. In other words, polarized light which should not be reflected by the PBS is reflected thereby, and polarized light which should be reflected thereby is transmitted therethrough without being reflected. As a result, in a state where a black image is projected, light (leakage light) which should be normally removed from projection light by reflection by the PBS is transmitted therethrough, and then is projected onto the black image, which reduces the contrast.
On the other hand, the spatial light modulating element has a similar angular characteristic thereto. That is, it has a characteristic which generates the leakage light projected onto the black image when an illumination optical system having a small F-number is used. Therefore, in order to project an image with a high contrast, it is necessary that an illumination light flux illuminating the spatial light modulating element be a collimated light flux (that is, a telecentric light flux with a large F-number).
Providing the aperture stop as disclosed in Japanese Patent Laid-Open No. 2004-45907 makes it possible to increase the F-number of the illumination optical system to obtain a high contrast. However, the limitation of the spread angle of light with the aperture stop blocks the light from the light source, which significantly reduces light utilization efficiency.
FIG. 6 shows an illumination optical system disclosed in Japanese Patent Laid-Open No. 2001-337393. The illumination optical system forms an illumination area with a homogeneous light amount distribution on a spatial light modulating element 54 by a first fly-eye lens 51a, a second fly-eye lens 51b and a condenser lens 52. When the spatial light modulating element 54 is disposed at a focal point position of the condenser lens 52 in this illumination optical system, the size of the illumination area is denoted with a magnification ratio B which is expressed by the following expression (7), the size of the illumination area corresponding to an enlarged size of each of lens cells constituting the first fly-eye lens 51a: B=fc/ff2  (7)where fc represents a focal length of the condenser lens 52, and ff2 represents a focal length of the second fly-eye lens 51b. 
Thus, when the focal length ff2 of the second fly-eye lens 51b is fixed, increasing the focal length fc of the condenser lens 52 increases the magnification ratio B, which can increase the F-number of the illumination optical system.
However, in order to make the light flux entering the spatial light modulating element 54 from the condenser lens 52 telecentric, it is necessary to set the focal point position of the condenser lens 52 calculated from the spatial light modulating element side at the vicinity of a light collection point of the first fly-eye lens 51a. 
Setting the light collection point of the first fly-eye lens 51a at the vicinity of the second fly-eye lens 51b makes it possible to increase utilization efficiency of the illumination light. In this case, the condenser lens 52 is disposed at an approximately intermediate position between the second fly-eye lens 51b and the spatial light modulating element 54. Reference numeral 55 denotes a principal point of the condenser lens 52.
When fc represents the focal length of the condenser lens 52, and Da represents an optical distance between the second fly-eye lens 51b and the spatial light modulating element 54, the following relationship is established:Da≈2fc  (8)
This means that the illumination optical system disclosed in Japanese Patent Laid-Open No. 2001-337393 has a very long entire length when the illumination optical system has a large F-number so as to achieve a high contrast.